Answer:
Positive real numbers
Step-by-step explanation:
The domain is all the possible values of x. Here, x represents the time that the ball falls.
Natural numbers are integers greater than 0 (1, 2, 3, etc.). However, the time doesn't have to be an integer (for example, x=1.5).
Positive integers are the same as natural numbers.
Positive rational numbers are numbers greater than 0 that can be written as a ratio of integers (1/1, 3/2, 2/1, etc.). However, the time can also be irrational (for example, x=√2).
Positive real numbers are numbers greater than 0 and not imaginary (don't contain √-1). This is the correct domain of the function.